The invention relates to the field of turbo-engines. It relates to a damper arrangement for reducing combustion-chamber pulsations in a gas turbine.
In the combustion of liquid or gaseous fuels in a combustion chamber of a gas turbine the so-called lean pre-mix combustion has become customary. In this case the fuel and the combustion air are pre-mixed as uniformly as possible and are then fed into the combustion chamber. In order to take account of ecological considerations, care is taken to have a low flame temperature by means of a substantial excess of air. In this way, the formation of nitrogen oxide can be kept low. A combustion chamber of a gas turbine with pre-mix burners is known for example from EP 387 532 A1.
In combustion chambers of this type, mutual building-up between thermal and acoustic interference results in so-called thermoacoustic oscillations which can thus assume large oscillation amplitudes in which the gas turbine reaches its limit of mechanical loading. In order to prevent this, dampers, by which the possible oscillation amplitudes are reduced or even eliminated, are provided in present-day gas-turbine combustion chambers.
By way of example, EP 597 138 B1 discloses an annular combustion chamber with burners and dampers which are secured inside the front plate of the annular combustion chamber and which are arranged alternately adjacent to one another in the peripheral direction. The dampers are accessible by way of a closable manhole in the external generated face of the annular combustion chamber and can thus be set manually in their damping frequency. This setting capacity is important since after the initial operation of a gas turbine the pulsation frequencies and the spatial formation of the combustion-chamber pulsations in the combustion chamber can be detected and suitable damping steps can be taken only under operating conditions. As is known, the damping to be achieved involves the damping of so-called noiseless components, in which individual frequency peaks in the noise spectrum should be reduced. The narrow-band oscillation excitations of high amplitude in the frequency range of from 50 to 600 Hz are typically found. The dampers used are so-called Helmholtz resonators and xcex/4 tubes which have to be tuned in terms of their damping frequency in accordance with the oscillation amplitude to be damped.
Intervention into the damping frequency of the dampers makes it necessary to uncover the gas turbine insofar as the opening of the annular combustion chamber and then the assembly of suitably tuned damping elements is possible. In terms of the shutdown of the machine this intervention into the gas turbine is correspondingly time-consuming and costly and it requires extreme care with respect to the operating technology, since no articles which could subsequently possibly lead to failure of the highly sensitive blade mounting of a machine at its loading limit can be allowed to remain in the gas turbine. Furthermore, the tuning of the damping frequency of the damping elements is possible only within specific limits. One restriction may be seen in the conditions of space which are available in the combustion chamber. In addition, the various combustion-chamber pulsations cannot be taken into consideration in their entire scope in different operating states of the gas turbine, such as full load or partial load, gas operation or oil operation in conjunction with a varying ambient temperature and different fuel/air ratios with the fixed installation of the dampers. In this way, frequency peaks can remain at particular loading points and operating states, and, although their effect is not immediately harmful, it is nevertheless desirable to reduce their level.
Although the damper installation known from the said EP 597 138 allows sufficiently satisfactory damping characteristics, it is limited in its flexibility in adjusting the gas turbine to changed situations in the overall system in a simple manner.
DE 196 40 980 likewise discloses a device for damping thermoacoustic oscillations in a combustion chamber, in which the damper arrangement comprises a Helmholtz resonator with a resonance volume and a damping tube. In order to achieve a greater damping performance the Helmholtz resonator is provided with a wall which is designed in the form of a mechanical spring. In addition, a mechanical mass, by which the virtual volume of the Helmholtz resonator is influenced, is arranged on this oscillating wall of the resonance volume. This known Helmholtz resonator is not readily accessible either for the purpose of subsequent adjustment of the damping frequency. This installation as well requires in fact correspondingly time-consuming and costly dismantling and assembly steps for tuning the damping frequencies.
The object of the invention is to provide a damper arrangement for reducing combustion-chamber pulsations arising inside a gas turbine, in such a way that it is possible to achieve improved damping characteristics by damper arrangements which are simple to install and easily accessible and the damping characteristics of which can, in addition, be set without substantial outlay. In this case it should be possible at least to set the damping frequencies without switching off or even uncovering the gas turbine. In addition, it should be possible to use relatively large damper volumes without substantial interference in known geometries of combustion chambers, these relatively large damper volumes having damping characteristics which were hitherto unattainable.
This object is attained as set out in claim 1. The damper arrangement according to the invention for a gas turbine is characterized in that further closable openings, through which damping elements can be inserted and tuned, are provided inside the turbine housing adjacent to the openings adapted to the burners. It is particularly advantageous that, in order to insert and/or tune a damping element, it is only necessary for this closable opening to be uncovered, which is possible in a more simple and rapid manner than in the case of the necessary steps on conventional gas-turbine plants. The damping elements can be inserted, as it were, from the outside through the turbine housing, without substantial areas of a gas turbine having to be uncovered in time-consuming and costly procedures, merely to allow access to the interior of the gas-turbine housing.
It is additionally important that the burners and the damping elements are interchangeable with one another, since the openings in a preferred embodiment for the burners and the openings for the damping elements are designed in an identical manner. Identically designed openings for burners and damping elements allow burners to be replaced by damping elements in the immediate vicinity of sites with increased pulsations in a combustion chamber and damping elements to be replaced by burners at sites with low thermoacoustic interference. This results in the greatest possible flexibility in effecting an optimum damping of combustion-chamber pulsations. In this way, the arrangement according to the invention has also made it possible to meet the long-standing requirement of providing a completely individual adaptation of a gas turbine in situ in a simple manner. As is known, only a detection of the combustion-chamber pulsations at various loading points can in fact be carried out after the initial operation. This procedure is performed in a particularly simple manner by damping elements which can be inserted and set from the outside and it permits an extremely rapid process in the tuning as a whole.
The openings for the burners in a front plate immediately towards the combustion chamber are advantageously arranged in such a way that the damping elements can also be flange-mounted on these openings. A distance is provided between the openings in the front plate and the closable openings in the turbine housing in such a way that the damping elements can be inserted therein completely.
A further advantageous arrangement of the invention provides that the damping elements project through the closable openings and out of the turbine housing. In this case the damping elements can be manipulated extremely easily from the outside, so that tuning of installed damping elements is possible in a simple manner even during the operation of the gas turbine. In this way, the tuning of the damping elements in the gas turbine can be carried out at different loading points, without the machine having to be shut down in the meantime. As a result, it is no longer necessary to carry out a time-consuming iterative procedure in order to move to specific loading points and subsequently to perform an associated tuning.
In a modem gas turbine with an annular combustion chamber the damping elements can occupy any position which a burner can also occupy, namely adjacent to one another radially or adjacent to one another in the peripheral direction.
It is advantageous for xcex/4 tubes and Helmholtz resonators to be used as the damping elements, which are additionally provided towards the outside with a tuning device which allows the damper volumes to be influenced directly.
Higher oscillation frequencies can typically be damped with xcex/4 tubes and lower oscillation frequencies with Helmholtz resonators, the frequency range of the thermoacoustic interference being limited between approximately 50 Hz at the bottom and approximately 600 Hz at the top.
In addition, it is possible to set each damping element by means of a tuning device whether the regulating circuit is opened or closed. In the case of a closed regulating circuit the oscillating frequencies of the combustion-chamber pulsations are fed directly to the said regulating circuit. The closed regulating circuit allows an automatic tuning of the damping elements, so that the damping frequencies are adapted as precisely as possible to the oscillating frequencies of the thermoacoustic interference at each operating point of the gas turbine.
In the case of an open regulating circuit, on the other hand, the damping elements can be set with external control and regulating variables.